On Earth, rain is just one part of the larger water cycle; it begins when water collected on the planet's surface is heated by the Sun. Some of which evaporates into the atmosphere, where it cools and condenses into clouds. Eventually, those clouds become heavy enough that gravity pulls the water back to Earth as rain.
In fact, on the Sun, coronal rain works similarly, but instead of 60-degree water, you're dealing with a million-degree plasma. Plasma, an electrically-charged gas, doesn't pool like water but instead traces the magnetic loops that emerge from the Sun's surface. Since at least the mid-1990s, scientists have thought that helmet streamers-million-mile tall magnetic loops named for their resemblance to a knight's pointy helmet-are one source of the slow solar wind, a dense stream of gas that escapes the Sun separately from its fast-moving counterpart.
Emily Mason, a graduate student at the Catholic University of America in Washington, D.C., and her coauthors published a paper in the Astrophysical Journal Letters describing the first observations of coronal rain. The findings forge a new link between the unusual heating of the corona and the source of the slow solar wind-two of the biggest mysteries facing solar science today.
But one part of the observations didn't coincide with previous theories. According to the current understanding, coronal rain only forms on closed loops, where the plasma can gather and cool without any means of escape. But as Mason sifted through data, she found cases where rain was forming on open magnetic field lines. Anchored to the Sun at only one end, the other end of these open field lines fed out into space, and plasma there could escape into the solar wind. To explain the inconsistency, Mason and the team developed an alternative explanation-one that connected rain on these tiny magnetic structures to the origins of the slow solar wind.
In the new explanation, the raining plasma begins its journey on a closed loop, but switches-through a process known as magnetic reconnection-to an open one. The phenomenon happens frequently on the Sun when a closed loop bumps into an open field line and the system rewires itself. Suddenly, the superheated plasma on the closed loop finds itself on an open field line. Some of that plasma will rapidly expand, cool down, and fall back to the Sun as coronal rain. But other parts of it will escape-forming one part of the slow solar wind.
Mason is currently working on a computer simulation of the new explanation, but she also hopes that soon-to-come observational evidence may confirm it. Now that Parker Solar Probe, launched in 2018, is traveling closer to the Sun than any spacecraft before it, it can fly through bursts of slow solar wind that can be traced back to the Sun-potentially, to one of Mason's coronal rain events.
